Optical frequency converter

ABSTRACT

An optical frequency converter that can use a low-amplitude, high-frequency signal for converting a wide range of optical frequencies. The optical frequency converter includes a device for modulating a lightwave of a preset frequency with a modulation signal to generate a group of sidebands thereof, a device for selecting sidebands from among the group of sidebands, and a device for changing modulation signal frequencies and selecting specific sidebands.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical frequency converter that canbe used as a variable-wavelength light source of optical routing devicesand the like used as switches in optical communications, and moreparticularly to an optical frequency converter that can be used as avariable-wavelength light source in which the optical wavelength rapidlystabilizes even when switched at high speed.

2. Description of the Prior Art

In the field of optical communications, methods of distributing inputoptical signals among a number of transmission lines include thefollowing: 1) a method in which the optical signals are converted intoelectrical signals and then re-transmitted as optical signals along thecorresponding transmission lines and 2) a method in which the opticalsignals are distributed as they are, with each signal being sent to thetransmission line concerned in accordance with differences in thewavelength of the light carrying the signal. It is also known that moreinformation is transmitted using the latter method.

The latter method is described, for example, in Reference (Aoyama etal., “Photonic Networks: Outlook and Technical Issues,” O plus E, vol.22, No. 11, November 2000, pp. 1456-1470). Optical signals input from aplurality of waveguides that have the same wavelength can be transmittedby a single waveguide by changing the carrier wavelength for each of thesignals input. This is a well-known method that is also described by theabove reference.

In the latter method in which the optical signals are distributed asthey are, a variable-wavelength light source is used. Thus, it can bereadily understood that it is desirable to use a variable-wavelengthlight source that is capable of high-speed operation and also has stablecharacteristics.

Light sources that have conventionally been used for this purposeinclude distributed Bragg reflection (DBR) lasers and distributedfeedback (DFB) lasers. However, after a wavelength has been changed, ittakes several tens of milliseconds for the output wavelength tostabilize, and in the case of 40-gigabit/second optical communicationsin which data is transmitted in 4000-bit packets, the time required tostabilize is longer than the time it takes to transmit one packet (100ns), posing an obstacle to such high-speed communications.

The present invention can be used for the above purpose, and relates toa variable-wavelength light source that uses a high-frequency signal toconvert an optical frequency. Prior-art examples of such a light sourceare described below.

There are a number of known ways converters work to convert thefrequency of an optical input. These include (1) the input of two typesof light to non-linear optical crystal to mix the two lightwaves. Thisis already well known, and is also used for doubling laser frequencies.Also included is (2) a method using a mode-locked laser, comprisingusing an optical modulator, isolator and Fabry-Perot etalon provided ina laser resonator to generate light pulses. This is also known as amethod for generating a sideband of frequency fp that is Km times higherthan phase modulation frequency fm (fp=Km*fm). There is also (3) amethod comprising modulating the light with a high-frequency signal toderive a sideband that is used to convert the optical frequency.

Using these methods, lightwaves are converted to different frequenciesas follows. In the case of the above (1), at least one of the lightwavesis changed to light of a different frequency. In the case of (2), afilter is also provided to select a generated sideband in order tochange the light to light of a different frequency. In the case of (3),the frequency of the high-frequency signal is changed. Thus, it canperhaps be readily seen that such methods can be used to change opticalfrequencies.

The present invention is partly similar to (3) in which the lightwave ismodulated by a high-frequency signal to obtain a sideband for convertingthe frequency. This is explained below.

Using a high-frequency signal to modulate a lightwave is usuallyaccomplished by inputting the optical carrier wave and high-frequencysignal to an optical modulator and performing intensity modulation orphase modulation or the like. With this method, when a sideband isobtained having a frequency higher than that of the appliedhigh-frequency signal, the high-frequency signal is multiplied,producing an electrical signal of an even higher frequency that is usedfor the modulation. Even when the high-frequency signal is thusmultiplied, the maximum modulation frequency is limited by the upperlimit of the electrical signal. Thus, multiplication or amplification ofan electrical signal has been subjected to the maximum frequencylimitation of the electrical circuit concerned. There has therefore beena need for a means of overcoming this.

There have been reports of using phase modulation with a high modulationindex in an attempt to obtain a sideband having a higher frequency thanthat of the applied high-frequency signal. One such report (Reference 1:“Generation of ultrashort light pulses using a domain-inversion externalphase modulator,” by Tetsuro Kobayashi, Applied Physics, vol. 67, No. 9(1998), pp. 1056-1060) stated that, with a modulation index of 87radian, applying a 16.26-GHz high-frequency signal to an opticalmodulator with a strip-line resonator provided on a waveguide formed ofelectro-optical LiTaO₃ crystal resulted in a spectral width of around2.9 THz.

Reference 2 (U.S. Pat. No. 5,040,865) describes the method of using ahigh-frequency signal to modulate monochromatic light with a modulatorhaving nonlinear characteristics, producing a high-order sideband that,by using an optical detector to detect the optical signal, is used toproduce a high-frequency signal. The disclosure describes using a firstmodulator to generate a first high-frequency signal by the above method,applying the signal to a second modulator to use the method to performmodulation with a second high-frequency signal. However, since this usesan electrical signal that is multiplied by an applied high-frequencysignal, it is subject to the frequency constraints of the circuit.

To perform the above-described phase modulation using a high modulationindex, it is necessary to realize a high modulation index. With thisbeing the aim, in order to increase the amplitude of the high-frequencysignal, a strip-line resonator is used as a modulator electrode, whichmakes it difficult to change the modulation frequency. If, to avoidthis, the resonator is not used as the electrode, a high-amplitudehigh-frequency signal becomes a requirement, so the high-frequencysignal is amplified. While it might seem that in this case, it is aneasy way of changing the optical frequency by changing the modulationfrequency, it is well known that the bandwidth of the amplifierdetermines upper frequency limit of the modulation signal and theobtained light frequency.

An object of the present invention is to provide an optical frequencyconverter that has a configuration that makes it possible to obtainhigh-order sidebands with a high-frequency signal having a loweramplitude than that of the above means of phase modulation using a highmodulation index, enabling conversion over a wide range of frequencieseven with a low-amplitude high-frequency signal.

SUMMARY OF THE INVENTION

To attain the above object, the first point of the present inventionrelates to an optical frequency converter comprising means formodulating light of predetermined frequency with a modulation signal toobtain a group of sidebands thereof, means for selecting sidebands fromamong the group of sidebands, and means for changing frequency of themodulation signal and selecting a predetermined sideband.

The object is also attained by the second point of the present inventionthat relates to an optical frequency converter comprising means formodulating light of predetermined frequency with a modulation signal toobtain an n−th order group of sidebands thereof where n is apredetermined integer of 1 or more, means for modulating the n−th ordergroup of sidebands to obtain an n+1−th order group of sidebands, meansfor selecting predetermined sidebands from among a group of numeroussidebands, and means for changing frequency of the modulation signal andchanging a predetermined sideband. Here, n−th order sideband means asideband that is separated from the carrier wave by a frequency amountthat is n times the modulation frequency, and n−th order group ofsidebands means two sidebands that are symmetrically positioned relativeto the carrier wave.

For performing multiplex modulation, the third point of the presentinvention relates to an optical frequency converter that in addition tothe first or second point can include reflecting means for folding anoptical path in the optical frequency converter.

For performing multiplex modulation, the fourth point of the presentinvention relates to an optical frequency converter that in addition tothe first, second or third point can also include modulation means to atleast one of which are input a group of different-order sidebands.

To form an optical circuit for performing multiplex modulation, thefifth point of the present invention relates to an optical frequencyconverter that in addition to the third point can also include a firstreflecting means that transmits light of the predetermined frequencyprior to modulation, and a second reflecting means having a plurality oftransmission bands.

To form an optical circuit for performing multiplex modulation, thesixth point of the present invention relates to an optical frequencyconverter that in addition to any one of the first to fifth points canalso include a first reflecting means comprised of a laser light sourceand a first narrow-bandpass filter, and a second reflecting meanscomprised of an optical modulator and a second narrow-bandpass filter.

In order to optimize the intensity of the output light, the seventhpoint of the present invention relates to an optical frequency converterthat in addition to any one of the first to sixth points can alsoinclude means for changing the length of an optical path of the opticalfrequency converter.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and followingdetailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a preferred embodiment of the opticalfrequency converter of the present invention.

FIG. 2 is a block diagram of another preferred embodiment of the opticalfrequency converter of the present invention.

FIG. 3 is a block diagram illustrating the basic principle of theoptical frequency converter.

FIG. 4 is a block diagram of a test apparatus for demonstrating theprinciple of the optical frequency converter.

FIG. 5 shows the relationship between sidebands, modulation frequencyand Fabry-Perot filter transmission spectrum.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of embodiments of the present invention will now be describedwith reference to the drawings. First, the principle of the inventionwill be explained, with reference to FIG. 3. In FIG. 3, the inputlightwave has a frequency f₀. A narrow bandpass filter 1 transmits theinput lightwave of frequency f₀ and reflects light having a slightlydifferent frequency than that of the input lightwave. The opticalmodulator is an intensity modulator that can modulate lightwavestraveling to the left or right with the same modulation frequency f_(m).A filter 2 is a narrow bandpass filter having a different pass-bandcenter to that of the filter 1, and has a smaller free spectral range(FSR) than the filter 1.

The input lightwave of frequency f₀ passes through the filter 1 and ismodulated, generating the sidebands shown in FIG. 3(b). For the sake ofsimplicity, modulation is assumed to be linear. It is also assumed thatthe original carrier wave disappears in the modulation and that only agroup of first-order sidebands is generated. The first-order sidebandsthat do not correspond to the transmission spectrums of the narrowbandpass filter 2 are reflected back through the intensity modulator, inthe course of which the sidebands undergo modulation that gives them thespectrum shown in FIG. 3(c). Components corresponding to the carrierwave are transmitted by the narrow bandpass filter 1, so just thesideband is reflected, as shown in FIG. 3(d), and is followed bymodulation, resulting in the spectrum of FIG. 3(e). This modulationgives rise to first-order and third-order sidebands. When the wavelengthof a third-order high-frequency sideband shown in FIG. 3(g) correspondsto the transmission spectral bands of the narrow bandpass filter 2, itis transmitted by the filter 2. In this way, a lightwave input from thenarrow bandpass filter 2 is output with a frequency that has beentripled by the high-frequency signal. Also, a sideband can be matched toa transmission spectrum of the narrow bandpass filter 2 by changing thefrequency of the high-frequency signal. Conversely, this means that thefrequency of the high-frequency signal and the requisite sideband ordercan be clarified by specifying the position from the center frequency ofthe transmission spectrum.

The above explanation has been made with reference to an opticalmodulator that is an intensity modulator, but the same effect can beobtained with a phase modulator. For the purpose of the presentinvention, a preferred modulator is a traveling-wave type modulator.With a traveling-wave type modulator, a lightwave traveling in eitherdirection can be modulated with the same characteristics by inputtingthe modulation signal via the electrode provided at each end.

FIG. 4 shows a test means for demonstrating the principle of theinvention. By means of reflection by a fiber grating (FBG) 1 and a fibergrating (FBG) 2, a lightwave input to the optical modulator isrepeatedly reflected back and forth to obtain high-order sidebands. Thesource laser is a semiconductor laser with a wavelength of 1550 nm andan output of 10 mW; the isolator is a commercial one manufactured byNewport Corporation. The gratings were made by 3M Company, and aredescribed, for example, in Reference 3 (Toru INOUE, “Development Trendsin Grating Technology,” C-3-67, Conference 2000 of The Institute ofElectronics, Information and Communications Engineers, pp. 246-247). Theoptical modulator is a traveling-wave type manufactured bySumitomo-Osaka Cement Co., Ltd. that can be operated by a high-frequencysignal input of up to 40 GHz. Using these components, it was possible toobtain a −32 dBm sideband output 210 GHz from the carrier wave byinputting a 30-GHz, 27.8 dBm modulation signal.

FIG. 1 shows a preferred embodiment of the optical frequency converterof the present invention. The optical frequency converter comprises asingle mode laser source 1 that emits light at a preset frequency (laserfrequency f_(LD)=200.033 THz), an isolator 2 to suppress the effect oflight traveling back, a polarization controller 3, a Fabry-Perot filter4 (with a transmission spectrum of 200.033 THz and a FSR of 300 GHz)forming a first reflecting means, an optical phase modulator 5 forperforming the modulation with a modulation signal and generatingsidebands, a Fabry-Perot filter 6 (with a transmission spectrum of200.000 THz and a FSR of 50 GHz) forming a second reflecting means, asplitter 7, an amplifier 8, and a high-frequency signal source 9. Thefrequency of the high-frequency signal source 9 can be changed, changingmodulation frequency, and constitutes the above sideband selectionmeans.

A lightwave from the single mode laser source 1 passes through theFabry-Perot filter 4 and is phase-modulated by the optical phasemodulator 5. Phase modulation produces high-order sidebands.

FIG. 5 shows the relationship between sidebands, modulation frequencyand the transmission spectrum of the Fabry-Perot filter 6. A modulationfrequency of 17 GHz results in a first-order sideband of frequency200.050 GHz. Since that is within the transmission spectrum of theFabry-Perot filter 6, it can pass through the filter 6, but thesecond-order sideband having a frequency of 200.067 GHz cannot passthrough the Fabry-Perot filter 6, and neither can the third-ordersideband having a frequency of 200.084 GHz.

A modulation frequency of 33.5 GHz results in a first-order sideband offrequency 200.0665 GHz. Since that is not within the transmissionspectrum of the Fabry-Perot filter 6, it cannot pass through the filter6, but the second-order sideband having a frequency of 200.100 GHz canpass through the Fabry-Perot filter 6. The third-order sideband of200.1335 GHz also cannot pass through the Fabry-Perot filter 6. Otherlightwaves can pass through the Fabry-Perot filter 6. Table 1 shows therelationship between the modulation frequency that produces this light,and the sideband order. TABLE 1 The order from Difference the centerbetween the frequency of transmission interest of the spectrum ofFabry-Perot interest and the filter 6 single mode Modulationtransmission laser source frequency spectrum n frequency (GHz) Sidebandorder k (GHz) 1 17 1 17.0 2 67 2 33.5 3 117 3 39.0 0 33 1 33.0 1 83 241.5 2 133 3 44.3 3 183 4 45.75

Thus, when the frequency of the single mode laser source 1 is fixed,lightwave frequencies can be instantaneously switched by selecting amodulation frequency and sideband order that match a transmissionspectrum of the Fabry-Perot filter 6. With respect to the positiveintegers n and k, taking k as sideband order, n as the order from thecenter frequency of the transmission spectrum of the Fabry-Perot filter6, f_(LD) as the frequency of the single mode laser source 1, f_(FP) asthe center spectral frequency of the Fabry-Perot filter 6, with f_(FSR)denoting the FSR frequency and f_(M) the modulation frequency, thefollowing relationships are obtained.f _(LD) +k×F _(M) =f _(FP) +n×f _(FSR), orf _(LD) −k×f _(M) =f _(FP) +n×f _(FSR), orf _(LD) +k×f _(M) =f _(FP) −n×f _(FSR), orf _(LD) −k×f _(M) =f _(FP) −n×f _(FSR).

So, each value can be set in accordance with these relationships.

Values thus obtained are stored in controller 10 and referred to, ifnecessary, to set k and modulation frequency f_(M) for a given n. Withrespect to the high-speed switching of modulation frequency f_(M), thereare existing high-frequency oscillators capable of switching within 10to 20 ns, and these can be used to realize the high-speed switching oflightwave frequencies.

An advantage of the present invention is that it is not necessary toprepare a high-frequency signal able to cover the lightwave frequencyrange, since the object can be attained using a high-frequency signalwith about one-fourth the range.

FIG. 2 shows another preferred embodiment of the optical frequencyconverter of this invention. This optical frequency converter comprisesa single mode laser source 1, an optical isolator 2, a polarizationcontroller 3, a Fabry-Perot filter 4 (with a transmission spectrum of200.033 THz and a FSR of 300 GHz), an optical phase modulator 5, avariable optical delay line 11 that can be used to externally controlthe length of the optical path, a Fabry-Perot filter 6 (with atransmission spectrum of 200.000 THz and a FSR of 50 GHz), a splitter 7,an amplifier 8, and a high-frequency signal source 9. The variableoptical delay line 11 can be constituted by a conventional means, suchas a prism or reflector, that is used to change the free-space opticalpath, or by using an optical fiber that is heated to employ thermalexpansion to change the length of the optical path, or the length of theoptical fiber can be mechanically changed by using a piezoelectricelement or a magnetostrictor.

The effect of the variable optical delay line 11 is that it optimizesthe optical output intensity by adjusting the length of the optical pathbetween the Fabry-Perot filter 4 and the Fabry-Perot filter 6. In thecourse of obtaining high-order sidebands by repeatedly reflecting thelightwave input to the optical modulator back and forth between theFabry-Perot filters 4 and 6, the intensity of the optical output dependson the phase of the light at the point of reflection. Since this lightphase depends on the phase of the light from the single mode lasersource 1, the modulation frequency and the length of the optical path,the length of the optical path is adjusted to optimize the intensity ofthe output lightwave. The variable optical delay line 11 is controlledby the controller 10, being switched to match the switching of thelightwave frequency. Thus, in the case of this embodiment, with respectto a given n, in addition to the aforementioned switching of k andf_(M), variable optical delay line 11 conditions are also changed.

As described in the foregoing, the variable optical delay line 11 isused to adjust the light phase to optimize the intensity of the outputlight by adjusting the optical path length. However, this can also beachieved by using a bias generator 12 controlled by the controller 10 toapply a bias voltage to the optical phase modulator 5 to thereby adjustthe phase. The advantage of using a bias voltage to optimize the outputintensity is the short response time. An advantage of using the variableoptical delay line 11 for the adjustment is that it can be used inhigh-noise-level environments. This embodiment uses a selector 13 toselect the adjustment means.

To ensure stable operation under changing ambient temperatureconditions, it is desirable to be able to externally control thetransmission spectrum characteristics of the narrow bandpass filter 2,via such means as voltage, current, temperature, magnetic field orelectromagnetic waves. This can be done by using the Fabry-Perot etalontype variable-wavelength filter in a cavity filled with dispersion typepolymer liquid crystal described, for example, in Reference 4 (JP-A HEI11-95184).

Reference 5 (Shimotsu and four others, “Subcarrier Generation byIntegrated Type LN Phase Modulator,” C-3-20, Conference 2000 of TheInstitute of Electronics, Information and Communications Engineers, p.199) describes a modulator that attenuates the carrier wave, leavingsidebands, which modulator can be used instead of the aforementionedmodulator.

The optical modulator can also be a semiconductor-based absorption type,a Mach-Zehnder type intensity modulator that uses a material having anelectro-optical effect, or a phase modulator having an electro-opticaleffect.

As long as the optical amplifier used in this means is located betweenthe narrow bandpass filters 1 and 2, its placement has no particularsignificance, with the same effect being forthcoming even if it and thevariable optical delay line 11 exchange places.

Each of the above embodiments has been described as using a Fabry-Perotfilter as the narrow bandpass filters 1 and 2. However, the filters donot have to be Fabry-Perot filters, it being also possible to use fiberBragg gratings, for example, to form the narrow bandpass filters. Anadvantage of using a fiber Bragg grating is that it enables the opticalpath to be configured inside the optical fiber, which makes it possibleto prevent loss of signal intensity during optical signal input/outputoutside the fiber. Another merit is that by changing the fiber Bragggrating structure, it is possible to form a filter having a differentperiod to that of a Fabry-Perot filter, in which the transmission bandperiods are equally spaced.

The present invention thus configured using the means described in theforegoing, provides the following effects.

The optical frequency converter configuration that includes means formodulating light of predetermined frequency with a modulation signal toobtain a group of sidebands thereof, means for selecting sidebands fromamong the group of sidebands, and means for changing frequency of themodulation signal and selecting a predetermined sideband makes itpossible to instantaneously switch optical signal frequencies byselecting the high-frequency signal frequency and sideband order.

Also, the use of repeated modulations makes it possible toinstantaneously switch optical signal frequencies with a smallermodulation signal.

Also, using reflecting means to configure a folded optical path makes itpossible to perform multiplex modulation.

Moreover, since repeated modulation operations can be performed with asingle modulator, the number of modulators used can be decreased.

Also, the second reflecting means having a plurality of transmissionbands that is used for sideband selection is an existing, well-knownoptical component, facilitating the configuration of an opticalfrequency converter that can switch optical frequencies.

The fact that the laser source, the reflecting means used to form thenarrow bandpass filters and the optical modulator are also existing,well-known means also makes it readily possible to realize an opticalfrequency converter that can switch lightwave frequencies.

In addition, in the seventh invention, the inclusion of means foradjusting the length of the optical path makes it possible to obtainoutput light of optimum intensity.

1. An optical frequency converter comprising: means for modulating lightof predetermined frequency with a modulation signal to obtain a group ofsidebands thereof; means for selecting sidebands from among the group ofsidebands; and means for changing frequency of the modulation signal andselecting a predetermined sideband. 2-4. (canceled)
 5. An opticalfrequency converter according to claim 1 that includes one or moremodulation means to at least one of which modulation means are input agroup of different order sidebands. 6-40. (canceled)